The present invention relates generally to electronic packages. More particularly, the present invention relates to an electronic package having an upper dielectric layer that dampens dispersion effects corresponding to the transmission of high speed signals.
An electronic package often includes a substrate having high speed signal traces formed on its upper surface. For example, a high speed transmission line can be realized as a coplanar waveguide (xe2x80x9cCPWxe2x80x9d) having signal and reference traces on the upper surface, a microstrip line having the signal trace on the upper surface and a reference plane below the upper surface, or a mixed mode structure that utilizes a CPW with an underlying reference plane. Routing high speed signals on the top layer of the package substrate is desirable because the signal need not experience a trace-to-via transition, which introduces a discontinuity and causes impedance mismatches. As a result of such signal routing, the dielectric constant of the material above the signal trace (i.e., air) may differ from the dielectric constant of the material below the signal trace (i.e., the package substrate material). Different dielectric constants lead to different phase velocities of electromagnetic wave propagation through the respective materials. This effect is known as dispersion, which adversely impacts the electrical performance of the high speed signal line. Dispersion increases as the difference between the dielectric constants increases. In practical applications, dispersion results in lost signal power, particularly at high frequencies. Consequently, dispersion can be problematic in some very high speed applications (e.g., OC-768 system and subsystem components) that require very low losses at frequencies up to 100 GHz.
An electronic package substrate configured in accordance with the techniques described herein includes an upper metal layer that defines a number of conductive traces. In one embodiment, the conductive traces form a CPW structure. These conductive traces are covered with material having a dielectric constant that is approximately equal to the dielectric constant of the underlying substrate material. The dielectric coating reduces the negative effects associated with dispersion.
The above and other aspects of the present invention may be carried out in one form by an electronic package component comprising a substrate formed from a material having a first dielectric constant, at least one conductive signal trace formed above the upper surface of the substrate, and a coating formed over the conductive signal trace, where the coating is formed from a material having a second dielectric constant that is approximately equal to the first dielectric constant.